1. Field of the Invention
The present invention relates generally to semiconductor devices, and, more particularly, to semiconductor devices that combine high-speed performance and low power consumption.
2. Description of the Related Art
An example of reducing power consumption by substrate bias control is described in 1996 IEEE International Solid-State Circuit, Digest of Technical Papers, (1996), pp. 166–167.
With the recent popularity of low-power CMOS LSIs (Complementary Metal Oxide Semiconductor Large Scale Integrated circuits), a trend has developed to maintain high-speed operation by decreasing the threshold voltage VT of the MOSFETs as the operating or supply voltage is dropped. When the supply voltage drops to 2 V or lower and when the threshold voltage VT is decreased to 0.5 V or lower correspondingly, however, the subthreshold leakage current increases, whereby the transistor cannot be cut off completely. Consequently, the standby current of the LSI chip increases, which represents a bottleneck in the design of a system that includes a battery-powered CMOS LSI chip. Furthermore, the current during normal operation also increases as the threshold voltage VT increases.
In order to break the bottleneck, a well-known system achieves a high-speed operation by decreasing the threshold voltage of each of the MOSFETs in the chip during normal operation, and decreases the standby current by increasing the threshold voltage at the time of standby. Nevertheless, the following three problems exist in this system:
(1) An overcurrent flows because of latching-up when the power supply is turned on, and the wiring in the CMOS LSI chip may fuse, or the normal supply voltage may become inapplicable as the load exceeds the current capacitance of the power supply. This problem is caused because the layout and connections of the circuit are designed so that the substrate (well) and source of the MOSFET are not at equipotential.
For example, when a p-channel MOSFET (PMOSFET) is used for applying a positive supply voltage (e.g., 1.8 V) to the source (p-layer), the pn junction between the source and well is excessively biased in the forward direction because the well (n-well) remains at a floating 0 V just until the application of the supply voltage, thus causing latch-up of the CMOS. In the case of conventional CMOS LSI products at 2 V or higher, the pn junction is never biased in the forward direction as in the normal operation thereafter, even during the application of the supply voltage, since the well and source of the MOSFET are connected so that both are at equipotential as much as possible. Since the threshold voltage VT is constant at all times at a value of substantially 0.5 or higher, moreover, there is no problem of subthreshold current.
In the case of an n-channel MOSFET (NMOSFET), the problem is not so serious. When the supply voltage is applied to the drain, the substrate (p-well) of the NMOSFET is at a floating 0 V and the source is fixed to an earth potential of 0 V, because the pn junction, formed between the drain and the well, is not biased in the forward direction. However, there is a subthreshold current flowing between the drain and source when the threshold voltage is 0.5 V or lower. By separately controlling the well and source, the threshold voltage in the CMOS LSI is lowered.
(2) The time required to switch the normal mode to the standby mode and the time required to switch the standby mode to the normal mode are extremely long, on the order of μs. Assuming that the substrate voltage is generated on-chip, by a charge pumping circuit for pumping the capacitor in the chip, the output current is limited to a low level. On the other hand, the transistor in the chip is used to connect the power supply terminals of the substrate in common, and consequently the total substrate capacitance has an extremely large value (100 pF or greater). Therefore, a large load (substrate) capacitance is driven by a substrate-voltage generating circuit whose current driving capability is low when the mode is switched, so that the response time tends to become longer.
(3) The subthreshold current flows everywhere, even in the CMOS circuit, thus increasing the operating current of the whole chip. This problem exists because, in the inactive state, the threshold voltage of the transistor in the CMOS circuit or circuit block is low during normal operation.